We wouldn't be able to remove signatures from the main chain retroactively

If you're going to rely on "social consensus" to define the validity of old blocks you can just create a new block ID scheme, and publish a checkpoint based on that, or even have one literal large BLOB+hash containing the entirety of millions of old blocks, and dispense with all other verification and data needed only for verification. It doesn't matter that old blocks didn't work that way, once you've decided to accept this snapshot going forward.